What should i use for the acne on my jaw/neck and chest? what is causing it to break out?

Hard to say/depends,: This is hard to determine as to the cause. It could be the food you eat, like sweets and cakes, or possibly hormones especially in teenage years. If treatment with any of the simple topical agents that you can get from the pharmacy do not work, a visit to your doctor, or dermatologist would be the best course of action. Best of luck.